When tagging someone on chat, the content of the message should determine the order of names listed.
For example when the words "ban" "unban" or "moderator" are mentioned, more than likely the user is trying to reach out to the Moderators of the channel.
In a similar approach, we can also track users of the last X amount of messages sent because the user may also be likely to be replying to a comment. If the content of the previous messages match what the user is typing, then they will likely tag that former person as well.
I see that a lot of ideas posted regarding chat are about improving this tagging experience, and what better way to improve it based on what the user has already written down. :)